A player character can become a 'being between worlds' (ergo a creature you can use summoning abilities on) via many different methods like the Chimera spell and so on.

My question is the following:

If a pair of summoners become 'beings between worlds' and both choose to make each other their respective familiars what would happen?

Would one be treated as "the summoner" and the other as "the familiar" or would they both be both at the same time? And if it is as in the first case what would happen if they made another ritual with inversed roles?

I am not sure on what the legal answer would be, but I do believe that bound/familiars cant regenerate zeon naturally. If that is the case, this is a very bad idea.

it is the case, familiar pull their zeon regen from their master's upkeep cost, so in that aspect, dual binding is very infeasable